1. Technical Field
The present invention relates to an internetwork device adapted to translate between local addresses used within a specific network and global addresses used across several networks.
2. Description of Related Art
Communications between a local network that use local addresses, and a global network that connect several networks and uses global addresses (herein termed “the internet” by way of example) take place through address translation between local addresses and global addresses, carried out in a router located at the boundary of the local network and the internet. Such address translation is known as Network Address Translation. Also, in order to effectively utilize a limited number of global addresses, a translation scheme that also utilizes TCP/UDP communication port numbers in addition to local addresses and global addresses (known as Network Address and Port Translation) is commonly used as well. Herein, “Address Translation” (also called “NAT”) is used to mean “Network Address and Port Translation”. Such address translation is employed in routers intended to connect small scale networks in household or businesses to the internet.
More recently, for reasons such as a shortage of global addresses available with IPv4, even among Internet Service Providers (hereinafter also called “ISP”) that serve large numbers of users there is an increasing need to accommodate users by allocating local addresses to internal network routers. In such instances, NAT functionality is required at the connection point of the ISP to the internet (the edge of the carrier network). NAT at this location differs significantly both in terms of role and required capabilities from NAT used to connect enterprise users and individual users to the ISP, and is called Large Scale NAT (LSN) or Carrier Grade NAT (CGN). Routers having LSN functionality are required to have so-called “carrier grade” performance and reliability. Routers having LSN functionality are also called “internetwork devices”.
In most instances higher reliability is required of LSN than of conventional NAT. Accordingly, LSN is sometimes implemented through a configuration in which two redundant internetwork devices operate simultaneously (called “Double ACT configuration”). Where LSN is implemented by internetwork devices in Double ACT configuration, the problem arises that fragmented packets, which are packets created by dividing a single packet (original packet) into multiple parts, may not be processed correctly. Specifically, if network load dispersion is carried out at random by a communication device upstream from the internetwork devices, it is possible that a set of fragmented packets created from a given original packet (hereafter called a “same-source fragmented packet group”) may not be collected by the same internetwork device. If a same-source fragmented packet group is not collected by the same internetwork device, it becomes difficult for the internetwork device to carry out identical address translation on each fragmented packet contained in the same-source fragmented packet group. Thus, the terminal that is the recipient of the fragmented packets may not recognize that the received fragmented packets belong to a same-source fragmented packet group, and may not be able to correctly reassemble the original packet. Also, depending on the load on each internetwork device, differences may arise in terms of latency from input to output of fragmented packets, leading to the possibility that fragmented packets are not forwarded in the correct sequence number to a device downstream from the internetwork devices.
Such problems are not limited to configurations with two internetwork devices, but are common to instances in which address translation is carried out in a configuration with multiple internetwork devices operating simultaneously.